Technical Field
The present invention relates to a solder paste for bonding micro components, which suppresses electro migration.
Background Art
So far a soldering alloy of Sn—Ag—Cu or Sn—Cu has been widely used which was proposed in Japanese patent No. 3027441 as so called Pb-free soldering alloy that is almost without Pb.
When soldering a micro component to a print circuit a reflow soldering method which uses a pasty soldering paste that is prepared by mixing a powdered soldering alloy and a pasty or liquid flux is widely adopted.
Electro-migration is a phenomenon where at a metal wiring portion or a solder bonding portion, diffusion of metal atoms occurs by an interaction between metal atoms and electrons running in the wired portion and metal atoms migrate toward a direction of an electric current. It is a well known fact that electro-migration occurs notably when a current density is high.
In the field of semiconductors refinement of wiring has been pursued due to rapid expansion of semiconductor demands. This caused occurrence of electro-migration through high current density in a metal wiring to make disconnection of the metal wiring, which became a major issue.
A solution for a disconnection of Aluminum wiring was proposed in a Japanese patent No. 3021996 which teaches a use of Aluminum wiring with uniformly added Scandium, whereby preventing an occurrence of electro-migration.
Meanwhile a possibility of occurrence of electro-migration at a solder bonding portion has been regarded as small as its area is larger than that of a metal wiring used in semiconductor field. However, the area of a solder bonding portion has become considerably smaller together with refinement of a circuit board and components on it and emergence of reflow soldering method as a main stream of soldering.
For example, a chip capacitor and a chip resistor are soldered through reflow soldering method using a solder paste. Recently sizes of a chip capacitors and a chip resistor are refined remarkably and models such as ‘0603’ with length of 0.6 mm and width of 0.3 mm, and ‘0402’ with length of 0.4 mm and width of 0.2 mm are widely used. Further, a model of ‘0201’ with length of 0.2 mm and width of 0.1 mm is now under development.
Under these circumstances where increased current density together with decreased area of a solder bonding portion is anticipated an occurrence of electro-migration at a solder bonding portion is concerned. When electro-migration occurs at a solder bonding portion there is a possibility that the soldering alloy would have a crack and an exfoliation and as a result electrical disconnection would occur. The disconnection at the solder bonding portion would cause a malfunction of an electronic device and may result in a fatal accident.